Fixing device

ABSTRACT

In a fixing apparatus using an induction heating type of heating apparatus, there are provided an excitation apparatus that generates magnetic flux, an opposed core located opposite the excitation apparatus, a fixing belt that is induction-heated by the aforementioned magnetic flux, and magnetism masking elements that block magnetic paths corresponding to paper non-passage areas of fixing belt between the excitation apparatus and the opposed core. By blocking magnetic paths between the excitation apparatus and the opposed core by the magnetism masking elements, magnetic flux that induction-heats the fixing belt is effectively masked, and an excessive rise in temperature of paper non-passage areas of fixing belt is prevented.

TECHNICAL FIELD

The present invention relates to a fixing apparatus useful foremployment in an image forming apparatus such as an electrophotographicor electrostatographic copier, facsimile machine, or printer, and moreparticularly to a fixing apparatus that heat-fixes an unfixed image ontoa recording medium using an induction heating type of heating section.

BACKGROUND ART

An induction heating (IH) type of fixing apparatus generates an eddycurrent through the action of a magnetic field generated by a magneticfield generation unit in a heat-producing element, and heat-fixes anunfixed image on a recording medium such as transfer paper or an OHPsheet through Joule heating occurring in the heat-producing element dueto this eddy current.

An advantage of this induction heating type of fixing apparatus comparedwith a heat roller type of fixing apparatus that uses a halogen lamp asa heat source is that heat production efficiency is higher and thefixing speed can be increased.

A fixing apparatus that uses a thin heat-producing element comprising athin sleeve or endless belt as the heat-producing element is known. Withsuch a fixing apparatus, the thermal capacity of the heat-producingelement is low and the heat-producing element can be made to produceheat in a short time, enabling startup responsiveness until heatproduction at a predetermined fixing temperature to be markedlyimproved.

On the other hand, with a fixing apparatus that uses this kind ofheat-producing element of low thermal capacity, heat is lost simplythrough the passage of a recording medium, causing a drop in temperatureof the paper passage area. Therefore, with this kind of fixingapparatus, the heat-producing element is heated in a timely fashion sothat the temperature of the paper passage area is maintained at apredetermined fixing temperature.

Consequently, with a fixing apparatus that uses this kind ofheat-producing element of low thermal capacity, if a recording medium ofsmall size is fed through continuously, the heat-producing element iscontinuously heated, and a phenomenon whereby the temperature of a papernon-passage area becomes abnormally higher than the temperature of apaper passage area-that is, a phenomenon of an excessive rise intemperature of a paper non-passage area-occurs.

A known technology for eliminating this kind of phenomenon of anexcessive rise in temperature of a paper non-passage area is onewhereby, of the magnetic flux generated by an exciting apparatus thatperforms induction heating of the heat-producing element, only magneticflux that acts on a paper non-passage area of the heat-producing elementis absorbed by a magnetic flux absorption member capable of moving inthe heat production width direction of the heat-producing element (see,for example, Patent Document 1).

Another known technology for eliminating the above-described phenomenonof an excessive rise in temperature of the paper non-passage area is onewhereby a second core of magnetic material corresponding to a papernon-passage area is positioned at the rear of a first core of magneticmaterial of a magnetic flux generation section that causes heatgeneration of a heat-producing element by electromagnetic induction, andthe lengthwise temperature distribution of the heat-producing element ischanged by varying the gap between the first core of magnetic materialand second core of magnetic material (see, for example, Patent Document2).

FIG. 1 is a schematic oblique drawing of an embodiment of a fixingapparatus disclosed in Patent Document 1. As shown in FIG. 1, thisfixing apparatus is provided with a coil assembly 10, a metal sleeve 11,a holder 12, a pressure roller 13, a magnetic flux masking shield 31, adisplacement mechanism 40, and so forth.

In FIG. 1, coil assembly 10 generates a high-frequency magnetic field.Metal sleeve 11 is heated by an induction current induced by aninduction coil 18 of coil assembly 10, and rotates in the direction oftransportation of recording material 14. Coil assembly 10 is held insideholder 12. Holder 12 is fixed to a fixing unit frame (not shown) anddoes not rotate. Pressure roller 13 rotates in the direction oftransportation of recording material 14 while pressing against metalsleeve 11 and forming a nip area. By having recording material 14gripped and transported by means of this nip area, an unfixed image onrecording material 14 is heat-fixed to recording material 14 by heatedmetal sleeve 11.

As shown in FIG. 1, magnetic flux masking shield 31 exhibits anarc-shaped curved surface that mainly covers the upper half of inductioncoil 18, and is advanced and withdrawn with respect to the gap at eitherend of coil assembly 10 and holder 12 by means of displacement mechanism40. Displacement mechanism 40 has a wire 33 linked to magnetic fluxmasking shield 31, a pair of pulleys 36 on which wire 33 is suspended,and a motor 34 that rotates one of the pulleys 36.

When the size of recording material 14 is the maximum size, magneticflux masking shield 31 is moved by means of displacement mechanism 40 soas to be withdrawn into the position shown by the solid line in FIG. 1.On the other hand, when the size of recording material 14 is small,magnetic flux masking shield 31 is moved so as to advance into theposition shown by the dot-dot-dash line in FIG. 1. By this means,magnetic flux reaching a paper non-passage area of metal sleeve 11 frominduction coil 18 is masked, and an excessive rise in temperature of apaper non-passage area is suppressed.

FIG. 2A and FIG. 2B are schematic cross-sectional views of a embodimentof a fixing apparatus disclosed in Patent Document 2. As shown in FIG.2A and FIG. 2B, this fixing apparatus is provided with a heatingassembly 51, a holder 52, a core-holding rotating member 53, an excitingcoil 54, a first core 55, a second core 56, a fixing roller 57, apressure roller 58, and so forth.

In FIG. 2A and FIG. 2B, heating assembly 51 is composed of holder 52,core-holding rotating member 53, exciting coil 54, first core 55, andsecond core 56, and generates magnetic flux. Fixing roller 57 isinduction-heated through the action of magnetic flux generated byheating assembly 51, and rotates in the direction of transportation ofrecording material 59.

Pressure roller 58 rotates in the direction of transportation ofrecording material 59 while pressing against fixing roller 57 andforming a nip area. By having recording material 59 gripped andtransported by means of this nip area, an unfixed image on recordingmaterial 59 is heat-fixed to recording material 59 by heated fixingroller 57.

First core 55 has the same width as the width of the maximum paperpassage area of fixing roller 57. When the size of recording material 59is the maximum size, second core 56 is moved to a position close tofirst core 55, as shown in FIG. 2A. On the other hand, when the size ofrecording material 59 is small, core-holding rotating member 53 rotatesthrough 180 degrees and second core 56 is moved to a position away fromfirst core 55, as shown in FIG. 2B. By this means, heat production of apaper non-passage area of fixing roller 57 corresponding to second core56 is suppressed.

Patent Document 1: Unexamined Japanese Patent Publication No.HEI10-74009

Patent Document 2: Unexamined Japanese Patent Publication No.2003-123961

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, as the fixing apparatus disclosed in Patent Documents 1 has aconfiguration whereby magnetic flux masking shield 31 is advanced andwithdrawn with respect to the gap at either end of coil assembly 10 andholder 12 by means of displacement mechanism 40, there is a problem inthat the pair of pulleys 36 of displacement mechanism 40 project greatlyfrom either end of holder 12, as shown in FIG. 1, and the body of thefixing apparatus is correspondingly large. Also, as shown in FIG. 1, afixing apparatus disclosed in Patent Document 1 has a configurationwhereby magnetic flux masking shield 31 is positioned between metalsleeve 11 formed of magnetic material and induction coil 18. In a fixingapparatus that uses induction heating, it is necessary to keep the gapbetween induction coil 18 and metal sleeve 11 narrow—on the order of 1mm, for example—to increase magnetic coupling. It is necessary formagnetic flux masking shield 31 to be made thin in order to be insertedin this narrow gap. That is to say, there is a problem of electricalresistance increasing because magnetic flux masking shield 31 cannot bemade sufficiently thick, and of magnetic flux masking shield 31 tendingto produce heat itself. Although heat production due to eddy currentscan be suppressed by forming through-holes 35 in magnetic flux maskingshield 31, magnetic flux reaches metal sleeve 11 as a result, and apaper non-passage area of the metal sleeve produces heat. As a result,there is a problem in that when small-size recording material 14 is fedthrough continuously, heat is accumulated in a paper non-passage area ofmetal sleeve 11, and an excessive rise in temperature cannot besuppressed.

Also, as shown in FIG. 2A and FIG. 2B, in a fixing apparatus disclosedin Patent Document 2, the distance between first core 55 and fixingroller 57 does not vary even though second core 56 is displaced withrespect to first core 55 by the rotation of core-holding rotating member53, and therefore the magnetic gap between a paper passage area andpaper non-passage area of fixing roller 57 is fixed.

Consequently, with this fixing apparatus, diverted flow of magnetic fluxfrom the end of the paper passage area corresponding to first core 55 tothe end of the paper non-passage area corresponding to second core 56occurs, and the efficacy of magnetic flux suppression in a paper passagearea of fixing roller 57 becomes low. As a result, a problem with thisfixing apparatus is that when small-size recording material 59 is fedthrough continuously, heat is accumulated in a paper non-passage area offixing roller 57, and an excessive rise in temperature cannot beeffectively suppressed.

Also, with this fixing apparatus, only a second core 56 for onerecording material size can be held in core-holding rotating member 53,and therefore the paper passage area width of fixing roller 57 can onlybe made to provide for two recording material paper widths—maximum sizeand small size.

It is therefore an object of the present invention to provide a smallfixing apparatus that can eliminate diverted flow of magnetic flux froma paper passage area of a heat-producing member to a paper non-passagearea, and prevent an excessive rise in temperature of the papernon-passage area.

MEANS FOR SOLVING THE PROBLEMS

A fixing apparatus of the present invention has: a magnetic fluxgeneration section that generates magnetic flux; a heat-producingelement of a nonmagnetic electrical conductor that allows passage of theaforementioned magnetic flux and is induction-heated; at least onemagnetism masking element that masks the aforementioned magnetic flux;and a magnetic flux adjustment section that switches between masking andclearing of magnetic flux with respect to a paper non-passage area ofthe aforementioned heat-producing element; wherein the aforementionedmagnetism masking element is located on the opposite side of theaforementioned heat-producing element from the aforementioned magneticflux generation section.

ADVANTAGEOUS EFFECTS OF THE INVENTION

According to the present invention, a compact apparatus can be achieved,and diverted flow of magnetic flux from a paper passage area of aheat-producing element to a paper non-passage area can be eliminated,enabling an excessive rise in temperature of the paper non-passage areato be prevented.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic oblique drawing showing the configuration of aconventional fixing apparatus;

FIG. 2A is a schematic cross-sectional view showing the configuration ofthe principal parts of another conventional fixing apparatus;

FIG. 2B is a schematic cross-sectional view illustrating the operationof another conventional fixing apparatus;

FIG. 3 is a schematic cross-sectional view showing the overallconfiguration of an image forming apparatus suitable for incorporationof a fixing apparatus according to Embodiment 1 of the presentinvention;

FIG. 4 is a cross-sectional view showing the basic configuration of afixing apparatus according to Embodiment 1 of the present invention;

FIG. 5 is a schematic cross-sectional view showing the configuration ofthe principal parts of a fixing apparatus according to Embodiment 1 ofthe present invention;

FIG. 6 is a schematic oblique drawing showing a configuration in whichmagnetism masking elements are provided on an opposed core of a fixingapparatus according to Embodiment 1 of the present invention;

FIG. 7 is a schematic oblique drawing showing the configuration of adisplacement mechanism that displaces magnetism masking elements of afixing apparatus according to Embodiment 1 of the present invention;

FIG. 8 is a schematic cross-sectional view showing a state in whichmagnetism masking elements of a fixing apparatus according to Embodiment1 of the present invention have been displaced to the magnetic pathblocking position;

FIG. 9 is a schematic cross-sectional view showing the configuration ofthe principal parts of a fixing apparatus according to Embodiment 2 ofthe present invention;

FIG. 10 is a schematic cross-sectional view showing the configuration ofthe principal parts of a fixing apparatus according to Embodiment 3 ofthe present invention;

FIG. 11 is a schematic cross-sectional view showing the configuration ofthe principal parts of a fixing apparatus according to Embodiment 4 ofthe present invention;

FIG. 12 is a schematic cross-sectional view showing the configuration ofa fixing apparatus according to Embodiment 5 of the present invention;

FIG. 13 is a schematic oblique drawing showing a configuration in whichmagnetism masking elements are provided on an opposed core of a fixingapparatus according to Embodiment 6 of the present invention;

FIG. 14 is a schematic oblique drawing showing the configuration of adisplacement mechanism that displaces magnetism masking elements of afixing apparatus according to Embodiment 6 of the present invention;

FIG. 15 is a schematic cross-sectional view showing a state in whichmagnetism masking elements of a fixing apparatus according to Embodiment6 of the present invention have been displaced to the magnetic pathblocking position;

FIG. 16 is a schematic cross-sectional view showing the configuration ofthe principal parts of a fixing apparatus according to Embodiment 7 ofthe present invention;

FIG. 17 is a schematic cross-sectional view showing the configuration ofthe principal parts of a fixing apparatus according to Embodiment 8 ofthe present invention;

FIG. 18 is a schematic oblique drawing showing the configuration of adisplacement mechanism that displaces cutaway parts of an opposed coreof a fixing apparatus according to Embodiment 8 of the presentinvention;

FIG. 19 is a schematic cross-sectional view showing the configuration ofthe principal parts of a fixing apparatus according to Embodiment 9 ofthe present invention;

FIG. 20 is a configuration diagram of principal parts in which anelectrical conductor is embedded in cutaway parts of an opposed core ofa fixing apparatus according to Embodiment 10 of the present invention;

FIG. 21 is a schematic cross-sectional view showing the configuration ofprincipal parts in which an electrical conductor is embedded in recessesof an opposed core of a fixing apparatus;

FIG. 22 is a schematic oblique drawing showing magnetism maskingelements of an opposed core corresponding to an A3 size recording paperpaper-passage mode of a fixing apparatus according to Embodiment 11 ofthe present invention;

FIG. 23 is a schematic cross-sectional view showing the configuration ofthe principal parts of a fixing apparatus with the opposed core shown inFIG. 22 cut through plane E;

FIG. 24 is a schematic oblique drawing showing magnetism maskingelements of an opposed core corresponding to a B4 size recording paperpaper-passage mode of a fixing apparatus according to Embodiment 11 ofthe present invention;

FIG. 25A is a schematic cross-sectional view showing the configurationof the principal parts of a fixing apparatus with the opposed core shownin FIG. 24 cut through plane F;

FIG. 25B is a schematic cross-sectional view showing the configurationof the principal parts of a fixing apparatus with the opposed core shownin FIG. 24 cut through plane G;

FIG. 26 is a schematic oblique drawing showing magnetism maskingelements of an opposed core corresponding to an A4 size recording paperpaper-passage mode of a fixing apparatus according to Embodiment 11 ofthe present invention;

FIG. 27A is a schematic cross-sectional view showing the configurationof the principal parts of a fixing apparatus with the opposed core shownin FIG. 26 cut through plane H;

FIG. 27B is a schematic cross-sectional view showing the configurationof the principal parts of a fixing apparatus with the opposed core shownin FIG. 26 cut through plane I;

FIG. 28 is a schematic oblique drawing showing magnetism maskingelements of an opposed core corresponding to an A5 size recording paperpaper-passage mode of a fixing apparatus according to Embodiment 11 ofthe present invention;

FIG. 29A is a schematic cross-sectional view showing the configurationof the principal parts of a fixing apparatus with the opposed core shownin FIG. 28 cut through plane J;

FIG. 29B is a schematic cross-sectional view showing the configurationof the principal parts of a fixing apparatus with the opposed core shownin FIG. 28 cut through plane K;

FIG. 30 is a schematic cross-sectional view showing the configuration ofthe principal parts of a fixing apparatus in which two magnetism maskingelements have lengths corresponding to A4 size width and B4 size widthpaper non-passage areas;

FIG. 31A is a schematic cross-sectional view showing the positions ofcutaway parts of an opposed core corresponding to an A3 size recordingpaper paper-passage mode of a fixing apparatus according to Embodiment11 of the present invention;

FIG. 31B is a schematic cross-sectional view showing the positions ofcutaway parts of an opposed core corresponding to a B4 size recordingpaper paper-passage mode of a fixing apparatus;

FIG. 31C is a schematic cross-sectional view showing the positions ofcutaway parts of an opposed core corresponding to an A4 size recordingpaper paper-passage mode of a fixing apparatus;

FIG. 32 is a schematic cross-sectional view showing the configuration ofthe principal parts of a fixing apparatus in which magnetism maskingelements are provided inside the opposed core shown in FIGS. 31A, 31B,and 31C.

FIG. 33 is a schematic cross-sectional view of principal parts showingthe configuration of a fixing apparatus according to Embodiment 12 ofthe present invention;

FIG. 34 is a schematic oblique drawing showing a paper passage areamagnetism masking element of an opposed core of a fixing apparatusaccording to Embodiment 12 of the present invention;

FIG. 35 is a schematic cross-sectional view showing the configuration ofa fixing apparatus according to Embodiment 13 of the present invention;

FIG. 36 is a schematic cross-sectional view showing the configuration ofa magnetic flux control mechanism of a fixing apparatus according toEmbodiment 13 of the present invention;

FIG. 37 is a schematic oblique drawing showing the configuration of amagnetic flux control section of a fixing apparatus according toEmbodiment 13 of the present invention;

FIG. 38 is a schematic cross-sectional view showing the configuration ofa supporting roller of a fixing apparatus according to Embodiment 14 ofthe present invention;

FIG. 39 is a schematic cross-sectional view showing the configuration ofanother supporting roller of a fixing apparatus according to Embodiment14 of the present invention;

FIG. 40 is a schematic cross-sectional view showing the configuration ofa supporting roller of a fixing apparatus according to Embodiment 15 ofthe present invention;

FIG. 41 is a schematic cross-sectional view showing the configuration ofa supporting roller of a fixing apparatus according to Embodiment 16 ofthe present invention;

FIG. 42 is a schematic oblique drawing showing a plate that is acomponent of a supporting roller of a fixing apparatus according toEmbodiment 16 of the present invention; and

FIG. 43 is a schematic cross-sectional view showing the configuration ofa fixing apparatus according to Embodiment 17 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The gist of the present invention is that a magnetism masking element isprovided that is located in a freely movable fashion between a magneticflux generation section and an opposed core and moves relative to theaforementioned magnetic flux generation section in the direction ofmovement of a heat-producing element that allows passage of magneticflux, and blocks or clears a magnetic path corresponding to a papernon-passage area of the aforementioned heat-producing element betweenthe aforementioned magnetic flux generation section and theaforementioned opposed core.

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings. In the drawings,configuration elements and equivalent parts that have identicalconfigurations or functions are assigned the same codes, anddescriptions thereof are not repeated.

Embodiment 1

FIG. 3 is a schematic cross-sectional view showing the overallconfiguration of an image forming apparatus suitable for incorporationof a fixing apparatus according to Embodiment 1 of the presentinvention.

As shown in FIG. 3, an image forming apparatus 100 has anelectrophotographic photosensitive body (hereinafter referred to as“photosensitive drum”) 101, an electrifier 102, a laser beam scanner103, a developing unit 105, a paper feed apparatus 107, a fixingapparatus 200, a cleaning apparatus 113, and so forth.

In FIG. 3, photosensitive drum 101 is rotated at a predeterminedperipheral velocity in the direction indicated by the arrow while itssurface is uniformly charged to a negative predetermined dark potentialV0 by electrifier 102.

Laser beam scanner 103 outputs a laser beam 104 modulated in accordancewith a time series electrical digital pixel signal of image informationinput from a host apparatus such as an image reading apparatus orcomputer (not shown), and performs scanning exposure of the surface ofuniformly charged photosensitive drum 101 with laser beam 104. By thismeans, the absolute value of the potential of exposed parts ofphotosensitive drum 101 falls and becomes a light potential VL, and anelectrostatic latent image is formed on the surface of photosensitivedrum 101.

Developing unit 105 is provided with a rotated developing roller 106.Developing roller 106 is positioned opposite photosensitive drum 101,and a thin layer of toner is formed on its peripheral surface. Adeveloping bias voltage with an absolute value smaller than darkpotential V0 of photosensitive drum 101 and larger than light potentialVL is applied to developing roller 106.

By this means, negatively charged toner on developing roller 106 adheresonly to light potential VL parts of the surface of photosensitive drum101, the electrostatic latent image formed on the surface ofphotosensitive drum 101 is developed, and an unfixed toner image 111 isformed on photosensitive drum 101.

Meanwhile, paper feed apparatus 107 feeds recording paper 109 as arecording medium one sheet at a time at predetermined timing by means ofa paper feed roller 108. Recording paper 109 fed from paper feedapparatus 107 is transported through a pair of registration rollers 110to the nip area between photosensitive drum 101 and a transfer roller112 at appropriate timing synchronized with the rotation ofphotosensitive drum 101. By this means, unfixed toner image 111 onphotosensitive drum 101 is transferred to recording paper 109 bytransfer roller 112 to which a transfer bias is applied.

Recording paper 109 on which unfixed toner image 111 is formed and heldin this way is guided by a recording paper guide 114 and separated fromphotosensitive drum 101, and then transported toward the fixing area offixing apparatus 200. Once transported to this fixing area, recordingpaper 109 has unfixed toner image 111 heat-fixed onto it by fixingapparatus 200.

After passing through fixing apparatus 200, recording paper 109 ontowhich unfixed toner image 111 has been heat-fixed is ejected onto anoutput tray 116 attached to the outside of image forming apparatus 100.

After recording paper 109 has been separated from it, photosensitivedrum 101 has residual material such as untransferred toner remaining onits surface removed by a cleaning apparatus 113, and is made ready forthe next image forming operation.

A fixing apparatus according to Embodiment 1 will now be described ingreater detail by giving a specific example. FIG. 4 is a cross-sectionalview showing the basic configuration of a fixing apparatus according toEmbodiment 1. As shown in FIG. 4, fixing apparatus 200 includes a fixingbelt 210, a supporting roller 220 serving as a belt supporting member,an excitation apparatus 230 serving as an induction heating mechanism, afixing roller 240, a pressure roller 250 serving as a belt rotationmechanism, and so forth.

In FIG. 4, fixing belt 210 is suspended between supporting roller 220and fixing roller 240. Supporting roller 220 is rotatably pivoted in theupper part of body side plate 201 of fixing apparatus 200. Fixing roller240 is rotatably pivoted in a rocking plate 203 attached in a freelyrocking fashion to body side plate 201 by means of a short shaft 202.Pressure roller 250 is rotatably pivoted in the lower part of body sideplate 201 of fixing apparatus 200.

Rocking plate 203 rocks in a clockwise direction about short shaft 202through the contracting action of a coil spring 204. Fixing roller 240is displaced in line with this rocking of rocking plate 203, and, bythis displacement, is pressed against pressure roller 250 with fixingbelt 210 inbetween. Supporting roller 220 is urged in the oppositedirection to fixing roller 240 by a spring (not shown), by which meanspredetermined tension is applied to fixing belt 210.

Pressure roller 250 is rotated in the direction indicated by the arrowby a driving source (not shown). Fixing roller 240 is rotated driven bythe rotation of pressure roller 250 while gripping fixing belt 210. Bythis means, fixing belt 210 is rotated in the direction indicated by thearrow, gripped between fixing roller 240 and pressure roller 250. Bymeans of this gripping and rotation of fixing belt 210, a nip area forheat-fixing unfixed toner image 111 onto recording paper 109 is formedbetween fixing belt 210 and pressure roller 250.

Excitation apparatus 230 comprises the above-described IH type inductionheating mechanism, and as shown in FIG. 4, has an exciting coil 231serving as a magnetism generation section installed along the outerperipheral surface of the part of fixing belt 210 suspended onsupporting roller 220, and a core 232 composed of ferrite coveringexciting coil 231. Exciting coil 231 extends in the paper passage widthdirection and is wound so as to loop back following the direction ofmovement of fixing belt 210. Inside supporting roller 220 is provided anopposed core 233 that is opposite exciting coil 231 with fixing belt 210and supporting roller 220 inbetween.

Exciting coil 231 is formed using litz wire comprising bundled thinwires, and the cross-sectional shape is formed as a semicircle so as tocover the outer peripheral surface of fixing belt 210 suspended onsupporting roller 220. An excitation current with a drive frequency of25 kHz is applied to exciting coil 231 from an exciting circuit (notshown). By this means, an alternating field is generated between core232 and opposed core 233, an eddy current is generated in the conductivelayer of fixing belt 210, and fixing belt 210 produces heat. In thisexample, the configuration is such that fixing belt 210 produces heat,but a configuration may also be used whereby supporting roller 220 ismade to produce heat, and heat from supporting roller 220 is transferredto fixing belt 210.

Core 232 is attached to the center and part of the rear of exciting coil231. As an alternative to ferrite, a high-permeability material such aspermalloy can also be used as the material of core 232 and opposed core233.

In fixing apparatus 200, as shown in FIG. 4, unfixed toner image 111 canbe heat-fixed onto recording paper 109 by transporting recording paper109 to which unfixed toner image 111 has been transferred from thedirection indicated by the arrow so that the surface bearing unfixedtoner image 111 is brought into contact with fixing belt 210.

A temperature sensor 260 comprising a thermistor is positioned at thepart of the rear surface of fixing belt 210 that has passed the area ofcontact with supporting roller 220. The temperature of fixing belt 210is detected by this temperature sensor 260. The output of temperaturesensor 260 is provided to a control apparatus (not shown). Based on theoutput of temperature sensor 260, this control apparatus controls thepower supplied to exciting coil 231 via the aforementioned excitingcircuit so that an optimal image fixing temperature is attained, and bythis means the calorific value of fixing belt 210 is controlled.

Downstream in the recording paper 109 transportation direction, an paperejection guide 270 that guides recording paper 109 toward output tray116 after heat-fixing is finished is provided in the area where fixingbelt 210 is suspended on fixing roller 240.

A coil guide 234 serving as a supporting member is also provided inexcitation apparatus 230, integral with exciting coil 231 and core 232.This coil guide 234 is formed of a resin with a high heat-resistancetemperature such as a PEEK material or PPS. The provision of coil guide234 makes it possible to confine heat emitted from fixing belt 210 inthe space between fixing belt 210 and exciting coil 231, and preventdamage to exciting coil 231.

Although core 232 shown in FIG. 4 has a semicircular cross-section, core232 need not necessarily have a shape that follows the shape of excitingcoil 231, and may, for example, have an approximately n-shapedcross-section.

Fixing belt 210 comprises, for example, a thin endless belt with adiameter of 50 mm and thickness of 50 μm, with a conductive layer formedby dispersing silver powder in base material of polyimide resin with aglass transition point of 360 (□). The conductive layer may be composedof 2 or 3 laminated silver layers with a thickness of 10 μm. The surfaceof this fixing belt 210 may be coated with a 5 μm thick release layer offluororesin (not shown) to provide releasability. It is desirable forthe glass transition point of the material of fixing belt 210 to be in arange from 200 (□) to 500 (□) Resin or rubber with good releasabilitysuch as PTFE, PFA, FEP, silicone rubber, fluororubber, or the like, maybe used, alone or mixed, for the release layer on the surface of fixingbelt 210.

As an alternative to the above-mentioned polyimide resin, aheat-resistant resin such as fluororesin or metal such as anelectroformed thin nickel sheet or thin stainless sheet can also be usedas the base material of fixing belt 210. For example, fixing belt 210may be configured by executing 10 μm thick copper plating on a 40 μmthick SUS430 (magnetic) or SUS304 (nonmagnetic) surface.

For performing heating control of fixing belt 210 in paper passage widthdirection (supporting roller 220 lengthwise direction) described laterherein, it is desirable for at least 50% of magnetic flux to passthrough fixing belt 210. It is therefore desirable for fixing belt 210to be formed using a nonmagnetic material such as silver or copper. Iffixing belt 210 is formed using a magnetic material, it should be madeas thin as possible (preferably not more than 50 μm thick). For example,if a 40 μm thick nickel belt is used, when excitation apparatus 230drive frequency f=25 kHz, a thickness of 40 μm is approximately half theskin depth of nickel (Ni), and approximately 60% of magnetic flux passesthrough fixing belt 210, facilitating heating control of fixing belt 210in the paper passage width direction.

When fixing belt 210 is used as an image heating element for heat-fixingof monochrome images, it is sufficient to secure releasability, but whenfixing belt 210 is used as an image heating element for heat-fixing ofcolor images, it is desirable for elasticity to be provided by forming athick rubber layer. The thermal capacity of fixing belt 210 shouldpreferably be 60 J/K or less, and still more preferably 40 J/K or less.

Supporting roller 220 is a cylindrical metal roller 20 mm in diameter,320 mm in length, and 0.2 mm thick. If the material of supporting roller220 is as thin as 0.04 mm or so, a magnetic material such as iron ornickel may be used, although a nonmagnetic material that allows easypassage of magnetic flux is preferable. The material should be asinsusceptible to the generation of eddy currents as possible, and use ofa nonmagnetic stainless material with a specific resistance of 50 μΩcmor higher is desirable. A supporting roller 220 of the nonmagneticstainless material SUS304 has a high specific resistance of 72 μΩcm aswell as being nonmagnetic, and therefore magnetic flux that passesthrough supporting roller 220 is not greatly masked, and with 0.2 mmthick material, for example, the heat production of supporting roller220 is extremely small. Also, a supporting roller 220 of SUS304 has goodmechanical strength, enabling the thermal capacity to be furtherdecreased by reducing the thickness to 0.04 mm, and is suitable for usein fixing apparatus 200 with this configuration. Supporting roller 220should preferably have a relative permeability of 4 or less, and be from0.04 mm to 0.2 mm in thickness.

Fixing roller 240 is 30 mm in diameter and made of silicone rubber, anelastic foam material with low surface hardness (here, JISA 30 degrees)and low thermal conductivity.

Pressure roller 250 is made of silicone rubber with a hardness of JISA65 degrees. A heat-resistant resin or other rubber such as fluororubberor fluororesin may also be used as the material of pressure roller 250.It is also desirable for the surface of pressure roller 250 to be coatedwith resin or rubber such as PFA, PTFE, or FEP, alone or mixed, toincrease wear resistance and releasability. Furthermore, it is desirablefor pressure roller 250 to be made of a material with low thermalconductivity.

As stated earlier, a problem with a conventional fixing apparatus ofthis kind is that, since the magnetic gap between a paper passage areaand paper non-passage area of the fixing belt is fixed, diverted flow ofmagnetic flux occurs from the edge of a paper passage area to a papernon-passage area, heat accumulates at the boundary between a paperpassage area and paper non-passage area of the fixing belt, a phenomenonof an excessive rise in temperature occurs at this boundary area, andthe body of the fixing apparatus is made larger. Also, with aconventional fixing apparatus, the paper passage area width of thefixing roller can only be made to provide for two recording materialpaper widths—maximum size and small size. In addition, there is aproblem of heat production by the magnetic flux masking shield thatmasks magnetic flux in a paper non-passage area.

Thus, as shown in FIG. 5, fixing apparatus 200 according to Embodiment 1has magnetism masking elements 301 of a material that can maskmagnetism. Magnetism masking elements 301 are located between excitationapparatus 230 and opposed core 233, and are supported so as to be freeto move relative to excitation apparatus 230 in the direction ofmovement of fixing belt 210 serving as a heat-producing element thatallows passage of magnetic flux.

In fixing apparatus 200 according to Embodiment 1, magnetism maskingelements 301 are configured so as to be displaced relative to excitationapparatus 230. A tubular sleeve (not shown) mated with opposed core 233,for example, can be used as a supporting member for magnetism maskingelements 301. In fixing apparatus 200 according to Embodiment 1, opposedcore 233 is used as a supporting member for magnetism masking elements301, as shown in FIG. 6.

The positions of magnetism masking elements 301 on opposed core 233 aredecided in accordance with the paper passage reference of recordingpaper 109. Here, the paper passage reference of recording paper 109 isassumed to be the center reference, and magnetism masking elements 301are provided at both ends of opposed core 233, as shown in FIG. 6. Ifthe maximum paper passage area width of fixing belt 210 corresponding tomaximum-size recording paper is designated A, and the small-size paperpassage area width of fixing belt 210 corresponding to small-sizerecording paper is designated B, as shown in FIG. 6, magnetism maskingelements 301 have a length C corresponding to the paper non-passage areaat either side of fixing belt 210 when small-size recording paper ispassed through.

In fixing apparatus 200 according to Embodiment 1, supporting roller 220comprises a member that does not mask, but allows passage of, magneticflux generated by excitation apparatus 230, such as the above-describednonmagnetic stainless material (SUS304) with a specific resistance of 72μΩcm, for example.

In FIG. 5, magnetism masking elements 301 are displaced to a magneticpath blocking position (the position shown by a dashed line in FIG. 5)in which they block a magnetic path 302 corresponding to a papernon-passage area of fixing belt 210 between excitation apparatus 230 andopposed core 233, and a magnetic path clearing position (the positionshown by a solid line in FIG. 5) in which they clear magnetic path 302.

FIG. 7 is a schematic oblique drawing showing a displacement mechanism500 that rotates opposed core 233 constituting the supporting member ofmagnetism masking elements 301 and displaces magnetism masking elements301. As shown in FIG. 7, this displacement mechanism 500 is composed ofa small gear wheel 501 attached to the spindle of opposed core 233, alarge gear wheel 502 that meshes with small gear wheel 501, an arm 503integral with the spindle of large gear wheel 502, a solenoid 504 thatcauses arm 503 to swing, and so forth.

In FIG. 7, when solenoid 504 is turned on (energized), the actuator ofsolenoid 504 moves and arm 503 swings. Through this swinging of arm 503,large gear wheel 502 rotates, and small gear wheel 501 rotates driven bythe rotation of large gear wheel 502. Through this driven rotation ofsmall gear wheel 501, the spindle of opposed core 233 rotates, andmagnetism masking elements 301 are displaced from the above-describedmagnetic path clearing position to the above-described magnetic pathblocking position shown in FIG. 8. By this means, magnetic paths 302corresponding to paper non-passage areas of fixing belt 210 betweenexcitation apparatus 230 and opposed core 233 are blocked by magnetismmasking elements 301.

On the other hand, when solenoid 504 in the above-described on state isturned off (de-energized), arm 503 returns to its initial position shownin FIG. 7, the spindles of large gear wheel 502, small gear wheel 501,and opposed core 233 are all rotated backward, and magnetism maskingelements 301 are returned from the above-described magnetic pathblocking position to the above-described magnetic path clearingposition.

Thus, in fixing apparatus 200 according to Embodiment 1, by turningsolenoid 504 of displacement mechanism 500 on and off, magnetic paths302 corresponding to paper non-passage areas of fixing belt 210 betweenexcitation apparatus 230 and opposed core 233 are blocked or cleared bymagnetism masking elements 301, and the strength of magnetic coupling inthe paper passage width direction between fixing belt 210 and excitingcoil 231 is controlled.

That is to say, when the size of recording paper 109 passed through isthe maximum size, solenoid 504 is left in the off state in FIG. 7, andmagnetism masking elements 301 are kept on standby in theabove-described magnetic path clearing position. As a result, as shownin FIG. 5, magnetic flux generated by excitation apparatus 230 flowsalong the entire lengthwise of opposed core 233 and acts on the whole ofmaximum paper passage area width A of fixing belt 210, and heatproduction distribution in the paper passage width direction of fixingbelt 210 is kept uniform over the whole of maximum paper passage areawidth A.

On the other hand, when the size of recording paper 109 passed throughis a small size, solenoid 504 is turned on in FIG. 7, and magnetismmasking elements 301 are displaced to the magnetic path blockingposition in which they block magnetic paths 302 corresponding to papernon-passage areas of fixing belt 210 between excitation apparatus 230and opposed core 233. As a result, magnetic coupling with exciting coil231 in paper non-passage areas of fixing belt 210 decreases, magneticflux generated by excitation apparatus 230 passes through onlysmall-size paper passage area width B of opposed core 233 shown in FIG.6, heat production of paper non-passage areas of fixing belt 210 issuppressed, and an excessive rise in temperature of these papernon-passage areas can be prevented.

In fixing apparatus 200 according to Embodiment 1, fixing belt 210 andmagnetism masking elements 301 are composed of a nonmagnetic electricalconductor such as silver, copper, or aluminum. As fixing belt 210 iscomposed of a thin nonmagnetic electrical conductor, its electricalresistance is high and it produces heat. Also since fixing belt 210 usesa nonmagnetic material, magnetic flux easily passes through fixing belt210. It is thus possible to provide magnetism masking elements 301 onthe opposite side of fixing belt 210 from excitation apparatus 230. Thatis to say, the necessity of making the magnetism masking elements thincan be eliminated, and their thickness can be increased to around 1 mm,for example. As a result, the electrical resistance of magnetism maskingelements 301 becomes low, enabling heat production by magnetism maskingelements 301 to be suppressed. Also, as magnetism masking elements 301are provided on opposed core 233 composed of a material with highthermal conductivity and specific heat such as ferrite, heat generatedby magnetism masking elements 301 is conducted and dispersed in opposedcore 233, and an excessive rise in temperature of magnetism maskingelements 301 is suppressed. Furthermore, increasing the thickness ofmagnetism masking elements 301 reduces their electrical resistance,making it easier for an eddy current to flow. As a result, a repulsivefield is strengthened, and magnetic flux can be more effectively masked.Also, since magnetism masking elements 301 do not require through-holes35, they can mask magnetic flux more effectively than magnetic fluxmasking shield 31 in FIG. 1.

As described above, in fixing apparatus 200 according to Embodiment 1,magnetic paths 302 between excitation apparatus 230 and opposed core 233are masked by magnetism masking elements 301, enabling magnetic flux ofpaper non-passage areas that induction-heats fixing belt 210 to beeffectively masked, and diverted flow of magnetic flux corresponding toa paper passage area of fixing belt 210 to paper non-passage areas to beprevented.

Thus, with fixing apparatus 200 according to Embodiment 1, magnetic fluxcorresponding to paper non-passage areas of fixing belt 210 can beeffectively blocked by magnetism masking elements 301, enabling anexcessive rise in temperature due to accumulation of heat in papernon-passage areas of fixing belt 210 to be prevented.

Also, in fixing apparatus 200 according to Embodiment 1, since magneticpaths 302 are blocked or cleared by relative displacement of excitationapparatus 230 and magnetism masking elements 301, the body of theapparatus need not be made large in the fixing belt 210 paper passagearea width direction.

Furthermore, in fixing apparatus 200 according to Embodiment 1, it ispossible to block magnetic flux corresponding to paper non-passage areasof fixing belt 210 by blocking only magnetic paths 302 betweenexcitation apparatus 230 and opposed core 233 by means of magnetismmasking elements 301, enabling magnetism masking elements 301 to beconfigured compactly, and making it possible for at least two magnetismmasking elements 301 to be provided. Therefore, in this fixing apparatus200, it is possible to make the paper passage area width of fixing belt210 accommodate at least three areas by providing magnetism maskingelements 301 having different lengths in the paper passage area widthdirection.

Moreover, in fixing apparatus 200 according to Embodiment 1, excitationapparatus 230 that heats fixing belt 210 directly is provided around theouter peripheral surface of the part of fixing belt 210 that issuspended on supporting roller 220. Therefore, in this fixing apparatus200, the permeability of supporting roller 220 itself is improved, andsupporting roller 220 does not become overheated even during continuousfixing, as a result of which the temperature difference between thetemperature of the paper passage area and the temperature of papernon-passage areas of fixing belt 210 due to thermal conduction fromsupporting roller 220 is within a permissible range, and the occurrenceof temperature unevenness in the paper passage width direction of fixingbelt 210 can be suppressed.

Also, as supporting roller 220 of fixing apparatus 200 according toEmbodiment 1 is a metal roller with a thickness of 0.04 mm to 0.2 mm,its thermal capacity is extremely small. Therefore, in this fixingapparatus 200, a large amount of heat is no longer lost from fixing belt210 due to contact with supporting roller 220 during warming-up, and thestartup time can be greatly shortened.

Furthermore, as supporting roller 220 of fixing apparatus 200 accordingto Embodiment 1 has a specific resistance of 50 μΩcm or higher, it isnot susceptible to the flow of eddy currents, heat production bysupporting roller 220 itself is virtually eliminated, and input powercan be effectively and efficiently used only for heat production offixing belt 210.

If a nonmagnetic stainless material (SUS304) with a specific resistanceof 72 μΩcm is used for supporting roller 220, magnetic flux passesthrough supporting roller 220 without being masked, and heat productionis extremely small even with 0.2 mm thick material. Also, thissupporting roller 220 has good mechanical strength and enables thestrength necessary for suspending fixing belt 210 to be secured,allowing the thermal capacity to be further decreased by reducing thethickness, and enabling the startup time to be further shortened duringwarming up.

When a supporting roller 220 of a nonmagnetic material with low specificresistance (such as aluminum or copper) is used, a large quantity ofeddy currents are generated by magnetic flux passing therethrough, andrepulsive fields are formed, with the result that magnetic flux crossingfixing belt 210 decreases and heat production efficiency falls. With asupporting roller 220 of a magnetic material with low specificresistance such as iron (Fe) or nickel (Ni), cross flux from fixing belt210 can be secured but supporting roller 220 produces heat itself due togenerated eddy currents, making startup slower.

Incidentally, the specific resistances (in μΩcm units) are as follows:9.8 for iron, 2.65 for aluminum, 1.7 for copper, 6.8 for nickel, 60 formagnetic stainless (SUS430), and 72 for nonmagnetic stainless (SUS304).

Embodiment 2

Next, a fixing apparatus according to Embodiment 2 will be described. Inthis fixing apparatus, as shown in FIG. 9, core 232 of excitationapparatus 230 has a center core 701 located in the center of thewindings of exciting coil 231. This fixing apparatus is configured sothat width W1 of magnetism masking elements 301 in the direction ofmovement relative to excitation apparatus 230 is greater than width W2of center core 701 in the same direction. As shown in FIG. 9, width W1of magnetism masking elements 301 and width W2 of center core 701 can bestipulated by angle θ1 and angle θ2.

By this means, with this fixing apparatus, in addition to the effects ofthe fixing apparatus of Embodiment 1, magnetic flux passing throughpaper non-passage areas of fixing belt 210 can be masked moreeffectively, and an excessive rise in temperature due to accumulation ofheat in paper non-passage areas of fixing belt 210 can be surelyprevented.

Embodiment 3

Next, a fixing apparatus according to Embodiment 3 will be described. Inthis fixing apparatus, as shown in FIG. 10, core 232 of excitationapparatus 230 has a shape with no center core. This fixing apparatus isconfigured so that width W1 of magnetism masking elements 301 in thedirection of movement relative to excitation apparatus 230 is greaterthan width W3 in the same direction of the center of the windings ofexciting coil 231 of excitation apparatus 230. Width W1 of magnetismmasking elements 301 and width W3 of the center of the windings ofexciting coil 231 can be stipulated by angles.

By this means, with this fixing apparatus, as with a fixing apparatusaccording to Embodiment 2, magnetic flux passing through papernon-passage areas of fixing belt 210 can be masked more effectively, andan excessive rise in temperature due to accumulation of heat in papernon-passage areas of fixing belt 210 can be surely prevented.

Embodiment 4

Next, a fixing apparatus according to Embodiment 4 will be described. Asshown in FIG. 11, this fixing apparatus is configured so that width W1of magnetism masking elements 301 in the direction of movement relativeto excitation apparatus 230 is greater than winding width W4 in the samedirection of a winding section of exciting coil 231.

By this means, with this fixing apparatus, in addition to realization ofthe effects of a fixing apparatus according to Embodiment 2 or a fixingapparatus according to Embodiment 3, as shown in FIG. 11, magnetismmasking elements 301 do not affect magnetic flux flowing in a magneticpath 302 formed by excitation apparatus 230 and opposed core 233 evenwhen the above-described magnetic path clearing position of magnetismmasking elements 301 is located opposite a winding section of excitingcoil 231.

That is to say, with this fixing apparatus, even if fixing belt 210 ismade to produce heat with magnetism masking elements 301 withdrawn to aposition opposite a winding section of exciting coil 231, temperatureunevenness does not occur in the paper passage area of fixing belt 210.Therefore, in this fixing apparatus, more withdrawal positions ofmagnetism masking elements 301 can be secured, and it is possible toincrease freedom of design when providing numerous magnetism maskingelements 301.

In all of the above-described fixing apparatuses according to Embodiment1 through Embodiment 4, a magnetic path blocking position at whichmagnetic paths 302 of paper non-passage areas of fixing belt 210 areblocked by magnetism masking elements 301 is assumed to be a position atwhich magnetism masking elements 301 are opposite the center of thewindings of exciting coil 231. This position opposite the center of thewindings of exciting coil 231 is the area in which magnetic flux betweenexciting coil 231 and opposed core 233 is most concentrated.

In the above-described fixing apparatuses according to Embodiment 1through Embodiment 4, since a position opposite the center of thewindings of exciting coil 231 where magnetic flux is most concentratedis used as the magnetic path blocking position of magnetism maskingelements 301, as stated above, an excessive rise in temperature of papernon-passage areas of fixing belt 210 can be effectively prevented.

Embodiment 5

Next, a fixing apparatus according to Embodiment 5 will be described.With this fixing apparatus, when a plurality of magnetism maskingelements 301 a, 301 b, and 301 c are provided as shown in FIG. 12, forexample, at least one magnetic path clearing position of these magnetismmasking elements is made a position at which magnetism masking elements301 are opposite a winding section of exciting coil 231.

With this fixing apparatus, when magnetism masking elements 301 a inFIG. 12 are in the above-described magnetic path clearing position,magnetic flux flowing in a magnetic path 302 formed by excitationapparatus 230 and opposed core 233 is not affected by magnetism maskingelements 301 a, and therefore even if fixing belt 210 is made to produceheat in this state, temperature unevenness does not occur in its paperpassage area.

Also, with this fixing apparatus, a location away from a winding sectionof exciting coil 231 can be made a magnetic path clearing position ofother magnetism masking elements 301 b and 301 c, facilitating theprovision of the plurality of magnetism masking elements 301 a, 301 b,and 301 c.

Embodiment 6

Next, a fixing apparatus according to Embodiment 6 will be described. Inthis fixing apparatus, a plurality of magnetism masking elements 301 a,301 b, and 301 c are provided for fixing belt 210 as shown in FIG. 13.These magnetism masking elements 301 a, 301 b, and 301 c have lengthscorresponding respectively to a plurality of paper non-passage areas offixing belt 210 with mutually different widths.

FIG. 14 is a schematic oblique drawing showing a displacement mechanism1200 that rotates opposed core 233 bearing the plurality of magnetismmasking elements 301 a, 301 b, and 301 c and displaces the plurality ofmagnetism masking elements 301 a, 301 b, and 301 c. As shown in FIG. 14,this displacement mechanism 1200 is composed of a small gear wheel 1201attached to the spindle of opposed core 233, a large gear wheel 1202that meshes with small gear wheel 1201, a stepping motor 1203 that isaxially connected to and rotates large gear wheel 1202, and so forth.

In FIG. 14, when stepping motor 1203 is turned on (energized), largegear wheel 1202 is rotated by the rotation of the spindle of steppingmotor 1203, and small gear wheel 1201 rotates driven by the rotation oflarge gear wheel 1202. Through this driven rotation of small gear wheel1201, the spindle of opposed core 233 rotate and, of magnetism maskingelements 301 a, 301 b, and 301 c, predetermined magnetism maskingelements of a length corresponding to the paper non-passage area widthof the passed-through recording paper size are displaced from theirmagnetic path clearing position to their magnetic path blockingposition. Here, magnetism masking elements 301 a are displaced fromtheir magnetic path clearing position to their magnetic path blockingposition as shown in FIG. 15. By this means, magnetic paths 302corresponding to paper non-passage areas of fixing belt 210 betweenexcitation apparatus 230 and opposed core 233 are blocked by magnetismmasking elements 301 a.

On the other hand, when the entire width of the paper passage area offixing belt 210 is made to produce heat, power to stepping motor 1203 iscut with magnetism masking elements 301 a, 301 b, and 301 c located intheir respective above-described magnetic path clearing positions asshown in FIG. 12.

Thus, in this fixing apparatus, by turning stepping motor 1203 ofdisplacement mechanism 1200 on and off, magnetic paths 302 correspondingto paper non-passage areas of fixing belt 210 between excitationapparatus 230 and opposed core 233 are blocked or cleared by magnetismmasking elements 301 a, 301 b, and 301 c, and the strength of magneticcoupling in the paper passage width direction between fixing belt 210and exciting coil 231 is controlled.

Therefore, with this fixing apparatus, by selectively displacingmagnetism masking elements 301 a, 301 b, and 301 c from theabove-described magnetic path clearing position to the magnetic pathblocking position in accordance with the size of recording paper passedthrough, heat production of paper non-passage areas of fixing belt 210corresponding to the size of recording paper 109 passed through issuppressed, enabling an excessive rise in temperature of papernon-passage areas of fixing belt 210 to be prevented. Therefore, withthis fixing apparatus, it is possible to achieve satisfactoryheat-fixing of a plurality of sizes of recording paper 109 by means offixing belt 210.

Embodiment 7

Next, a fixing apparatus according to Embodiment 7 will be described. Inthis fixing apparatus, as shown in FIG. 16, a plurality of magnetismmasking elements 301 a, 301 b, and 301 c are provided on opposed core233, which is a rotating element that rotates freely relative toexcitation apparatus 230, and the angle forming a normal line passingthrough the centers of two mutually adjacent magnetism masking elementsis set to an angle of either 30 degrees<θ3<60 degrees or 120degrees<θ4<180 degrees.

That is to say, in this fixing apparatus, as shown in FIG. 16,aforementioned angle θ3 between a magnetism masking element 301 b and amagnetism masking element 301 c is set to 30 degrees<θ3<60 degrees, andaforementioned angle θ4 between a magnetism masking element 301 a and amagnetism masking element 301 b is set to 120 degrees<θ4<180 degrees.

In this fixing apparatus, when plurality of magnetism masking elements301 a, 301 b, and 301 c are located in their respective above-describedmagnetic path clearing positions, magnetic flux flowing in a magneticpath 302 formed by excitation apparatus 230 and opposed core 233 is notaffected by any of plurality of magnetism masking elements 301 a, 301 b,and 301 c, and therefore it is possible to suppress the occurrence oftemperature unevenness of a paper passage area when fixing belt 210 ismade to produce heat in this state.

Here, it is desirable for above-described magnetism masking elements 301a, 301 b, and 301 c to be composed of a low-permeability electricalconductor. In a fixing apparatus in which these magnetism maskingelements 301 a, 301 b, and 301 c are composed of a low-permeabilityelectrical conductor, magnetism masking elements 301 a, 301 b, and 301 ccan be configured as inexpensive members of copper, aluminum, or thelike.

Also, as fixing apparatuses according to the above-described embodimentsuse opposed core 233 as a rotating element that supports magnetismmasking elements 301 a, 301 b, and 301 c, the configuration can besimplified.

Embodiment 8

Next, a fixing apparatus according to Embodiment 8 will be described. Inthis fixing apparatus, as shown in FIG. 17, the aforementioned magnetismmasking elements are configured as cutaway parts 1501 provided inopposed core 233. This cutaway parts 1501 of fixing apparatus aredisplaced to the above-described magnetic path blocking position ormagnetic path clearing position by means of displacement mechanism 500shown in FIG. 18 in accordance with the size of recording paper 109passed through. The same displacement mechanism 500 as shown in FIG. 7can be used as this displacement mechanism 500. A configuration is alsopossible in which cutaway parts functioning as magnetism maskingelements are provided in positions corresponding to magnetism maskingelements 301 a, 301 b, and 301 c shown in FIG. 16 instead of a singlepair of cutaway parts being provided.

In this fixing apparatus, since magnetic flux passes through supportingroller 220, by selectively reversing the position of cutaway parts 1501provided in opposed core 233 in accordance with the size of recordingpaper 109, magnetic flux that passes through supporting roller 220 canbe absorbed or suppressed, enabling heat production distribution in thepaper passage width direction of fixing belt 210 to be controlledeasily.

Also, with this fixing apparatus, since cutaway parts 1501 serving asabove-described magnetism masking elements need not be provided asseparate members, the configuration can be made simpler and lessexpensive.

Embodiment 9

Next, a fixing apparatus according to Embodiment 9 will be described. Inthis fixing apparatus, as shown in FIG. 19, the aforementioned magnetismmasking elements are configured as recesses 1701 provided in opposedcore 233. With this fixing apparatus, as with a fixing apparatusaccording to Embodiment 8, recesses 1701 serving as above-describedmagnetism masking elements need not be provided as separate members,enabling the configuration to be made simpler and less expensive.

With this fixing apparatus, as shown in FIG. 19, recesses 1701 do notaffect magnetic flux flowing in a magnetic path 302 formed by excitationapparatus 230 and opposed core 233 even when the magnetic path clearingposition of the magnetism masking elements is made a position opposite awinding section of exciting coil 231. Therefore, with this fixingapparatus, temperature unevenness does not occur in the paper passagearea of fixing belt 210 even if fixing belt 210 is made to produce heatwith recesses 1701 withdrawn to a position opposite a winding section ofexciting coil 231, and therefore more withdrawal positions of recesses1701 can be secured.

Embodiment 10

Next, a fixing apparatus according to Embodiment 10 will be described.As shown in FIG. 20, this fixing apparatus is configured with alow-permeability electrical conductor 1801 a embedded in above-describedcutaway parts 1501. It also has a configuration in which alow-permeability electrical conductor 1801 b is embedded inabove-described recesses 1701 as shown in FIG. 21.

With this fixing apparatus, a decrease in the mechanical strength ofopposed core 233 due to the provision of cutaway parts 1501 or recesses1701 can be prevented. Also, embedding electrical conductor 1801 a or1801 b in the cutaway parts 1501 or recesses 1701, enables equilibriumof the weight balance of opposed core 233 to be achieved.

Here, it is desirable for above-described electrical conductor 1801 a or1801 b to form the same single plane with the surface of opposed core233. In a fixing apparatus with a configuration in which electricalconductor 1801 a or 1801 b is flush with the surface of opposed core 233in this way, thermal conduction from fixing belt 210 to opposed core 233and thermal conduction from fixing belt 210 to electrical conductor 1801a or 1801 b are equal, and therefore the occurrence of temperatureunevenness of fixing belt 210 can be prevented.

Embodiment 11

Next, a fixing apparatus according to Embodiment 11 will be described.In this fixing apparatus, the above-described three magnetism maskingelements 301 a, 301 b, and 301 c have lengths corresponding respectivelyto A4 size width, A5 size width, and B4 size width paper non-passageareas of fixing belt 210.

Therefore, this fixing apparatus can be configured, for example, withthe provision of four paper-passage modes: a paper-passage mode of A3size recording paper 109 as shown in FIGS. 22 and 23, a paper-passagemode of B4 size as shown in FIG. 24 and FIGS. 25A and 25B, apaper-passage mode of A4 size as shown in FIG. 26 and FIGS. 27A and 27B,and a paper-passage mode of A5 size as shown in FIG. 28 and FIGS. 29Aand 29B.

That is to say, in a paper-passage mode of A3 size recording paper 109,magnetism masking elements 301 a, 301 b, and 301 c are all withdrawn tothe above-described magnetic path clearing positions as shown in FIG.23. As a result, magnetic path 302 is not blocked by any of magnetismmasking elements 301 a, 301 b, or 301 c, and a paper passage area of theentire width (A3 size width) of fixing belt 210 is heated. Here, FIG. 23is a cross-sectional view showing the opposed core shown in FIG. 22 cutthrough plane E.

In a paper-passage mode of B4 size recording paper 109, the shortest ofmagnetism masking elements 301 a, 301 b, and 301 c—that is, magnetismmasking elements 301 c—are positioned at the above-described magneticpath blocking position as shown in FIGS. 25A and 25B. As a result,magnetic path 302 is blocked by magnetism masking elements 301 c, andonly a paper passage area of fixing belt 210 corresponding to a B4 sizewidth is heated. Magnetism masking elements 301 a and 301 b are bothwithdrawn to their magnetic path clearing positions, preventingtemperature unevenness due to them within the paper passage area. Here,FIG. 25A is a cross-sectional view showing the opposed core shown inFIG. 24 cut through plane F, and FIG. 25B is a cross-sectional viewshowing the opposed core shown in FIG. 24 cut through plane G.

In A4 size recording paper 109 paper-passage mode, the medium-lengthmagnetism masking elements among magnetism masking elements 301 a, 301b, and 301 c—that is, magnetism masking elements 301 a—are positioned atthe above-described magnetic path blocking position as shown in FIGS.27A and 27B. As a result, magnetic path 302 is blocked by magnetismmasking elements 301 a, and only a paper passage area of fixing belt 210corresponding to an A4 size width is heated. Magnetism masking elements301 b and 301 c are both withdrawn to their magnetic path clearingpositions, preventing temperature unevenness due to them within thepaper passage area. Here, FIG. 27A is a cross-sectional view showing theopposed core shown in FIG. 26 cut through plane H, and FIG. 27B is across-sectional view showing the opposed core shown in FIG. 26 cutthrough plane I.

In a paper-passage mode of A5 size recording paper 109, the longest ofmagnetism masking elements 301 a, 301 b, and 301 c—that is, magnetismmasking elements 301 b—is positioned at the above-described magneticpath blocking position as shown in FIGS. 29A and 29B. As a result,magnetic path 302 is blocked by magnetism masking elements 301 b, andonly a paper passage area of fixing belt 210 corresponding to an A5 sizewidth is heated. Magnetism masking elements 301 a and 301 c are bothwithdrawn to their magnetic path clearing positions, preventingtemperature unevenness due to them within the paper passage area. Here,FIG. 29A is a cross-sectional view showing the opposed core shown inFIG. 28 cut through plane J, and FIG. 29B is a cross-sectional viewshowing the opposed core shown in FIG. 28 cut through plane K.

As shown in FIG. 30, two magnetism masking elements 1801 c and 1801 dmay also have lengths corresponding respectively to A size width and B4size width paper non-passage areas. With such an embodiment, sincemagnetism masking elements 1801 c and 1801 d form the same plane withthe surface of opposed core 233, thermal conduction from fixing belt 210to opposed core 233 and thermal conduction from fixing belt 210 tomagnetism masking elements 1801 c and 1801 d are equal, and theoccurrence of temperature unevenness of fixing belt 210 can beprevented. Also, magnetism masking element width W1 (circumferentiallength) can be made greater than when three magnetism masking elementsare used. That is to say, magnetic flux passing through papernon-passage areas of fixing belt 210 can be masked more effectively, andan excessive rise in temperature due to accumulation of heat in papernon-passage areas of fixing belt 210 can be prevented more surely.

The above-described paper-passage modes can also be supported by afixing apparatus in which the above-described magnetism masking elementsare configured as cutaway parts 1501 or recesses 1701. FIGS. 31A, 31B,and 31C are schematic cross-sectional views illustrating threepaper-passage modes when the aforementioned magnetism masking elementsare configured as two cutaway parts 1501 a and 1501 b.

In FIGS. 31A, 31B, and 31C, if cutaway parts 1501 a are taken ascorresponding to magnetism masking elements 301 a, and cutaway parts1501 b are taken as corresponding to magnetism masking elements 301 c,in A3 size recording paper 109 paper-passage mode, cutaway parts 1501 aand 1501 b are all withdrawn to the above-described magnetic pathclearing positions as shown in FIG. 31A. As a result, magnetic path 302is not blocked by cutaway parts 1501 a or 1501 b and a paper passagearea of the entire width (A3 size width) of fixing belt 210 is heated.

Also, in a paper-passage mode of B4 size recording paper 109, theshortest of cutaway parts 1501 a and 1501 b—that is, cutaway parts 1501b—are positioned at the above-described magnetic path blocking positionas shown in FIG. 31B. As a result, magnetic path 302 is blocked bycutaway parts 1501 b, and only a paper passage area of fixing belt 210corresponding to a B4 size width is heated.

Furthermore, in a 109 paper-passage mode of A4 size recording paper 109,the longest of cutaway parts 1501 a and 1501 b—that is, cutaway parts1501 a—are positioned at the above-described magnetic path blockingposition as shown in FIG. 31C. As a result, magnetic path 302 is blockedby cutaway parts 1501 a, and only a paper passage area of fixing belt210 corresponding to an A4 size width is heated.

According to this fixing apparatus, it is possible to perform continuousheat-fixing of A3 size images and A4 size images as business documents,and continuous heat-fixing of B4 size images as official documents andschool teaching materials, enabling this fixing apparatus to be used asa fixing apparatus of a multifunctional image forming apparatus.

As shown in FIG. 32, a tubular magnetism masking element 301 may also beprovided inside opposed core 233 shown in FIGS. 31A, 31B, and 31C. Withsuch an embodiment, magnetism masking element 301 faces center core 701via cutaway parts 1501 b provided in opposed core 233 by rotatingopposed core 233 to be at predetermined position, as shown in FIG. 32,enabling magnetic flux to be masked more efficiently. In this samplevariation, magnetism masking element 301 need not move, and maytherefore be fixed. Also, in this sample variation, an example has beendescribed in which a magnetic path is blocked or cleared by rotatingopposed core 233, but this is not a limitation, and a magnetic alloythat loses its magnetic properties when its temperature rises may beused instead of opposed core 233. If the temperature of papernon-passage areas of fixing belt 210 rises and the temperature of themagnetic alloy exceeds the Curie point, the magnetic properties of papernon-passage area of the magnetic alloy are lost, and magnetic paths ofpaper non-passage areas are blocked by magnetism masking element 301.With this sample variation, blocking and clearing of magnetic path areperformed automatically, which has the effect of making displacementmechanism 500 unnecessary.

Embodiment 12

Next, a fixing apparatus according to Embodiment 12 will be described.As shown in FIG. 33 and FIG. 34, this fixing apparatus has aconfiguration in which a paper passage area magnetism masking element2401 with a length corresponding to a paper passage area width smallerthan the maximum paper passage area width of fixing belt 210 is providedin a position corresponding to the paper passage area of fixing belt210.

In this fixing apparatus, paper non-passage areas can be made to rise intemperature by blocking magnetic path 302 with paper passage areamagnetism masking element 2401. If the temperature of paper non-passageareas of fixing belt 210 for which heat production has been prevented byabove-described magnetism masking elements 301 becomes too low, thetemperature can be raised to a predetermined fixing temperature in ashort time by means of paper passage area magnetism masking element2401.

Embodiment 13

Next, a fixing apparatus according to Embodiment 13 of the presentinvention will be described. FIG. 35 is a schematic cross-sectional viewshowing the configuration of a fixing apparatus according to Embodiment13 of the present invention. In a fixing apparatus 300 according toEmbodiment 13, supporting roller 220 is configured as a member that doesnot mask but allows passage of magnetic flux generated by excitationapparatus 230, composed of the aforementioned nonmagnetic stainlessmaterial (SUS304) with a specific resistance of 72 μΩcm, for example. Asshown in FIG. 35, this fixing apparatus 300 is provided with magneticflux control sections 310 that control heat production distribution inthe paper passage width direction (lengthwise direction) of fixing belt210 by absorbing or repelling magnetic flux that has passed throughsupporting roller 220.

As shown in FIG. 36 and FIG. 37, these magnetic flux control sections310 are located inside supporting roller 220, and are configured byproviding a small-size width control member 311 corresponding to asmall-size paper (for example, A4) size recording paper width, andmaximum width control members 312 corresponding to the maximum-sizepaper (for example, A3) size recording paper width, on a switchovershaft 313.

Small-size width control member 311 and maximum width control members312 comprise ferrite cores, and small-size width control member 311shown in the drawings is configured as a cylinder with a perfectlycircular cross-section. Maximum width control members 312 shown in thedrawings comprise ferrite cores with a fan-shaped cross-section withcutaway parts 312 a provided in part of the axial direction.

A magnetic flux control section 310 is not limited to the configurationof this embodiment, and it is possible to use a configuration in whichan electrical conductor such as aluminum or copper is embedded in acutaway part of a maximum width control member 312, and magnetic flux inthis part is reduced more effectively, or a configuration in which itemsthat absorb or repel magnetic flux—such as items in which an aluminum orcopper sheet is provided only in a part corresponding to a cutawaypart—are combined as appropriate without using a ferrite core.

The position of a small-size width control member 311 and a maximumwidth control member 312 on switchover shaft 313 is decided inaccordance with the recording paper 109 paper passage reference Forexample, when the recording paper 109 paper passage reference is acenter reference, a small-size width control member 311 is provided inthe center of switchover shaft 313 and a maximum width control member312 is provided at either side of small-size width control member 311 asshown in FIG. 35 and FIG. 36.

Switchover shaft 313 is rotated through a predetermined angle (in theexample shown in the drawings, an angle of approximately 180 degrees) bydisplacement mechanism 500 shown in FIG. 37 in accordance with the sizeof recording paper 109 passed through. Displacement mechanism 500 shownin the drawing is composed of a small gear wheel 501 attached toswitchover shaft 313, a large gear wheel 502 that meshes with small gearwheel 501, an arm 503 integral with the spindle of large gear wheel 502,a solenoid 504 that causes arm 503 to swing, and so forth.

In FIG. 37, when solenoid 504 is turned on (energized), the actuator ofsolenoid 504 moves and arm 503 swings. Through this swinging of arm 503,large gear wheel 502 rotates, and small gear wheel 501 rotates driven bylarge gear wheel 502. Through this driven rotation of small gear wheel501, switchover shaft 313 rotates, and the position of cutaway parts 312a of maximum width control members 312 is inverted by approximately 180degrees. When solenoid 504 is turned off (de-energized) in this state,arm 503 returns to its initial position, large gear wheel 502, smallgear wheel 501, and switchover shaft 313 are all rotated backward, andcutaway parts 312 a of maximum width control members 312 are restored totheir original position.

Thus, in magnetic flux control section 310 of fixing apparatus 300according to Embodiment 13, by turning solenoid 504 of displacementmechanism 500 on and off, the position of cutaway parts 312 a of maximumwidth control members 312 is inverted, and the strength of magneticcoupling in the paper passage width direction between fixing belt 210and exciting coil 231 is controlled.

That is to say, when the size of recording paper 109 passed through isthe maximum size, solenoid 504 is left in the off state in FIG. 37, andboth small-size width control member 311 and maximum width controlmembers 312 are made to face exciting coil 231 of excitation apparatus230. Consequently, as shown in FIG. 35 and FIG. 36, magnetic flux thatis generated by excitation apparatus 230 and passes through supportingroller 220 is absorbed over whole of maximum paper passage width Lm ofsupporting roller 220 by small-size width control member 311 and maximumwidth control members 312 and acts on the entire maximum paper passagewidth of fixing belt 210, and heat production distribution in the paperpassage width direction of fixing belt 210 is kept uniform over theentire maximum paper passage width.

On the other hand, when the size of recording paper 109 passed throughis a small size, solenoid 504 is turned on in FIG. 37, the position ofmaximum width control members 312 is reversed so that the position oftheir cutaway parts 312 a is opposite to exciting coil 231, and onlysmall-size width control member 311 corresponding to a small-sizerecording paper width is made to face exciting coil 231 of excitationapparatus 230. Consequently, as shown in FIG. 36, magnetic flux that isgenerated by excitation apparatus 230 and passes through supportingroller 220 is well absorbed over small-size paper passage width Ls ofsupporting roller 220 only by small-size width control member 311 andacts only on the small-size paper passage width of fixing belt 210. As aresult, magnetic coupling with exciting coil 231 in paper non-passageareas of fixing belt 210 decreases, heat production of paper non-passageareas of fixing belt 210 is suppressed more than heat production ofsmall-size paper passage width “Ls” of fixing belt 210, and an excessiverise in temperature of paper non-passage areas of fixing belt 210 can beprevented.

Thus, with fixing apparatus 300 according to Embodiment 13, sincesupporting roller 220 allows passage of magnetic flux, by selectivelyreversing the position of cutaway parts 312 a of maximum width controlmembers 312 in accordance with the size of recording paper 109, magneticflux passing through supporting roller 220 can be partially increased ordecreased, and heat production distribution in the paper passage widthdirection of fixing belt 210 can easily be controlled.

Embodiment 14

Next, a fixing apparatus according to Embodiment 14 will be described.FIG. 38 and FIG. 39 are schematic cross-sectional views showing theconfiguration of a supporting roller of a fixing apparatus according toEmbodiment 14 of the present invention.

As shown in FIG. 38, a configuration is used for a supporting roller 620of a fixing apparatus according to Embodiment 14 in which a thin metalsheet is formed into a cylindrical shape, and a joint 621 is welded.This fixing apparatus can be configured inexpensively since a weldedtube is used as its supporting roller 620.

Also, as shown in FIG. 39, an item in which rib-shaped reinforcinggrooves 721 formed along the direction of the generating line of thecylinder can be used as a supporting roller 720 of this Embodiment. Inthis fixing apparatus, supporting roller 720 can be configured with highbending strength using a thin material of small thermal capacity. Forexample, a supporting roller with small thermal capacity and highbending strength can be formed by forming rib-shaped reinforcing grooves721 even when a thin material not exceeding 100 μm in thickness is used.

However, in the case of supporting roller 620 configured as a weldedtube as shown in FIG. 38, joint 621 and non-joint parts have differentthermal capacities, and therefore temperature unevenness occurs in itssurface temperature. Also, in the case of supporting roller 720 on whichrib-shaped reinforcing grooves 721 are formed as shown in FIG. 39, theamount of thermal conduction from fixing belt 210 is different for apart touching fixing belt 210 and a part not touching fixing belt 210,and therefore temperature unevenness occurs in its surface temperature.

Thus, a fixing apparatus according to Embodiment 14 is configured sothat the circumference of fixing belt 210 is not an integral multiple ofthe circumference of supporting roller 620 or supporting roller 720. Ina fixing apparatus with this configuration, the rotational period offixing belt 210 is different from the rotational period of supportingroller 620 or supporting roller 720, and the point of contact betweensupporting roller 620 or supporting roller 720 and fixing belt 210during rotation of fixing belt 210 changes successively. Therefore,according to a fixing apparatus with this configuration, even iftemperature unevenness occurs in supporting roller 620 or 720, since theheat of supporting roller 620 or 720 is not conducted to and accumulatedat a fixed place, the surface temperature of fixing belt 210 can besmoothed so as to be free of unevenness.

Embodiment 15

Next, a fixing apparatus according to Embodiment 15 will be described.FIG. 40 is a schematic cross-sectional view showing the configuration ofa supporting roller of a fixing apparatus according to Embodiment 15 ofthe present invention.

As shown in FIG. 40, a supporting roller 820 of a fixing apparatusaccording to Embodiment 15 is configured by forming knurl-shapedprojections and depressions 821 on the outer surface of a cylinder. Withthis fixing apparatus, the area of contact between supporting roller 820and fixing belt 210 can be minimized.

Therefore, with a fixing apparatus according to Embodiment 15, thermalinsulation between fixing belt 210 and supporting roller 820 can beimproved, there is little loss of heat production energy of fixing belt210 during warming-up, and the startup time can be shortened.

However, with supporting roller 820 on which projections and depressions821 are formed in this way, if the rotational period coincides betweenpitch P of these projections and depressions 821 and fixing belt 210,the point of contact between projections and depressions 821 ofsupporting roller 820 and fixing belt 210 during rotation of fixing belt210 will always be the same, and temperature unevenness will occur inthe surface temperature.

Thus, a fixing apparatus according to Embodiment 15 is configured sothat the circumference of fixing belt 210 is not an integral multiple ofpitch P of projections and depressions 821.

In a fixing apparatus with this configuration, since the circumferenceof fixing belt 210 is not an integral multiple of pitch P of projectionsand depressions 821, the point of contact between supporting roller 820and fixing belt 210 during rotation of fixing belt 210 changessuccessively. Therefore, according to this fixing apparatus, even iftemperature unevenness occurs in the surface temperature of supportingroller 820, the heat of supporting roller 820 is not accumulated at afixed point of fixing belt 210, and the surface temperature of fixingbelt 210 can be smoothed so as to be free of unevenness.

Embodiment 16

Next, a fixing apparatus according to Embodiment 16 will be described.FIG. 41 is a schematic cross-sectional view showing the configuration ofa supporting roller of a fixing apparatus according to Embodiment 16 ofthe present invention.

As shown in FIG. 41, a supporting roller 920 of a fixing apparatusaccording to Embodiment 16 is configured, for example, by combining aplurality of sheets 921 comprising channel-shaped thin metal sheets suchas shown in FIG. 42 into a cylindrical shape.

In a fixing apparatus configured in this way, since supporting roller920 is configured using a plurality of sheets 921 comprisingchannel-shaped thin metal sheets, supporting roller 920 can be given aconfiguration with small thermal capacity and high bending strength.Also, according to this fixing apparatus, the outer diameter ofsupporting roller 920 can easily be changed by changing the quantity ofsheets 921 making up supporting roller 920.

Embodiment 17

Next, a fixing apparatus according to Embodiment 17 will be described.FIG. 43 is a schematic cross-sectional view showing the configuration ofa fixing apparatus according to Embodiment 17 of the present invention.

As shown in FIG. 43, in a fixing apparatus 1100 according to Embodiment17, a belt supporting member over which fixing belt 210 is suspended isconfigured, for example, as a guide member 1120 in which a sheetcomprising a thin metal sheet is formed into an arc shape.

With this image forming apparatus 1100, the space occupied by guidemember 1120 constituting a belt supporting member is smaller than whenthe aforementioned belt supporting member is configured as a supportingroller, enabling the circumference of fixing belt 210 to be minimized.Also in this fixing apparatus 1100, guide member 1120 constituting abelt supporting member can be configured with smaller thermal capacityand less expensively than in the case of the aforementioned supportingroller. This guide member 1120 may be configured, for example, bycutting away part of supporting roller 920 configured with a pluralityof sheets 921 comprising channel-shaped thin metal sheets shown in FIG.42.

The supporting rollers shown in above-described Embodiment 13 throughEmbodiment 17 can be applied to a heating apparatus other than a fixingapparatus of an image forming apparatus.

The present application is based on Japanese Patent Application No.2003-358024, filed on Oct. 17, 2003, Japanese Patent Application No.2003-358330, filed on Oct. 17, 2003, and Japanese Patent Application No.2004-155165, filed on May 25, 2004, the entire content of which isexpressly incorporated herein by reference.

INDUSTRIAL APPLICABILITY

A fixing apparatus according to the present invention enables divertedflow of magnetic flux from a paper passage area of a heat-producingmember to a paper non-passage area to be eliminated and an excessiverise in temperature of that paper non-passage area to be prevented,without enlarging the apparatus, and is therefore useful as a fixingapparatus of an electrophotographic or electrostatographic copier,facsimile machine, printer, or the like.

1. A fixing apparatus comprising: a magnetic flux generation sectionthat generates magnetic flux; a heat-producing element made of anonmagnetic electrical conductor, that allows passage of the magneticflux and is induction-heated; at least one magnetism masking elementthat masks the magnetic flux; and a magnetic flux adjustment sectionthat switches between masking and clearing of magnetic flux with respectto a paper non-passage area of said heat-producing element, wherein saidmagnetism masking element is located on the opposite side of saidheat-producing element from said magnetic flux generation section. 2.The fixing apparatus according to claim 1, comprising an opposed corelocated on the opposite side of said heat-producing element from saidmagnetic flux generation section, wherein said magnetism masking elementmoves relative to said magnetic flux generation section in a directionof movement of said heat-producing element, and is displaced between amagnetic path blocking position at which a magnetic path correspondingto a paper non-passage area of said heat-producing element between saidmagnetic flux generation section and the opposed core is blocked, and amagnetic path clearing position at which the magnetic path is cleared.3. The fixing apparatus according to claim 1, wherein: saidheat-producing element is formed in a circular shape; and said magnetismmasking element is located inside said heat-producing element; and saidmagnetic flux generation section is located outside said heat-producingelement.
 4. The fixing apparatus according to claim 1, wherein: saidmagnetic flux generation section comprises: an exciting coil that iswound and placed; and a center core located in a center of windings ofthe exciting coil; and a width of said magnetism masking element in adirection of movement relative to said magnetic flux generation sectionis greater than a width of the center core in the same direction.
 5. Thefixing apparatus according to claim 4, wherein the width of saidmagnetism masking element in a direction of movement relative to themagnetic flux generation section is narrower than a winding width of awinding section of the exciting coil in the same direction.
 6. Thefixing apparatus according to claim 5, wherein at least one magneticpath clearing position of said magnetism masking element is a positionat which said magnetism masking element is opposite the winding sectionof the exciting coil.
 7. The fixing apparatus according to claim 4,wherein a magnetic path blocking position at which a magnetic path of apaper non-passage area of said heat-producing element is blocked by saidmagnetism masking element is a position at which said magnetism maskingelement is opposite the center of the windings of the exciting coil. 8.The fixing apparatus according to claim 1, wherein: said magnetic fluxgeneration section comprises an exciting coil that is wound and placed;and a width of said magnetism masking element in a direction of movementrelative to said magnetic flux generation section is greater than awidth of a center of windings of the exciting coil in the samedirection.
 9. The fixing apparatus according to claim 8, wherein thewidth of said magnetism masking element in a direction of movementrelative to said magnetic flux generation section is narrower than awinding width of a winding section of the exciting coil in the samedirection.
 10. The fixing apparatus according to claim 9, wherein atleast one magnetic path clearing position of said magnetism maskingelement is a position at which said magnetism masking element isopposite the winding section of the exciting coil.
 11. The fixingapparatus according to claim 8, wherein a magnetic path blockingposition at which a magnetic path of a paper non-passage area of saidheat-producing element is blocked by said magnetism masking element is aposition at which said magnetism masking element is opposite the centerof the windings of the exciting coil.
 12. The fixing apparatus accordingto claim 1, comprising a plurality of said magnetism masking elementshaving lengths corresponding to each of a plurality of paper non-passageareas of mutually different widths of said heat-producing element. 13.The fixing apparatus according to claim 12, wherein: the plurality ofsaid magnetism masking element are provided on a rotating element thatrotates freely relative to said magnetic flux generation section; and anangle forming a normal line passing through centers of two mutuallyadjacent magnetism masking elements is set to an angle of either 30degrees<θ3<60 degrees or 120 degrees<θ4<180 degrees.
 14. The fixingapparatus according to claim 1, further comprising an opposed corelocated opposite said magnetic flux generation section, wherein saidmagnetism masking element is provided on the opposed core that isrotatable relative to said magnetic flux generation section.
 15. Thefixing apparatus according to claim 2, wherein said magnetism maskingelement is formed by a cutaway part provided in the opposed core. 16.The fixing apparatus according to claim 2, wherein said magnetismmasking element is formed by a recess provided in the opposed core. 17.The fixing apparatus according to claim 15, wherein an electricalconductor is embedded in the cutaway part.
 18. The fixing apparatusaccording to claim 17, wherein the electrical conductor forms a sameplane with a surface of the opposed core.
 19. The fixing apparatusaccording to claim 13, wherein an electrical conductor is embedded inthe recess.
 20. The fixing apparatus according to claim 19, wherein theelectrical conductor is formed flush with a surface of the opposed core.21. The fixing apparatus according to claim 1, wherein the plurality ofsaid magnetism masking elements have lengths corresponding to each of A3size width, A4 size width, and B4 size width paper non-passage areas ofsaid heat-producing element.
 22. The fixing apparatus according to claim1, comprising a paper passage area magnetism masking element having alength corresponding to a paper passage area width smaller than a widthof a maximum paper passage area of said heat-producing element, whereinthe paper passage area magnetism masking element is placed in a positioncorresponding to a paper passage area of said heat-producing element.23. The fixing apparatus according to claim 1, wherein: saidheat-producing element is configured with an endless belt; and a beltsupporting member on which the endless belt is suspended is configuredwith a member that allows passage of magnetic flux.
 24. The fixingapparatus according to claim 23, wherein the belt supporting member ismade of a metallic material with a thickness in a range of 0.04 mm to0.2 mm in a vertical direction with respect to a peripheral surface ofthe endless belt.
 25. The fixing apparatus according to claim 23,wherein the belt supporting member has a specific resistance of 50 μΩcmor more.
 26. The fixing apparatus according to claim 23, wherein thebelt supporting member is made of a nonmagnetic stainless material. 27.The fixing apparatus according to claim 23, wherein the belt supportingmember comprises a rotatable supporting roller in which a sheet isformed into a cylindrical shape and a joint is welded.
 28. The fixingapparatus according to claim 23, wherein the belt supporting membercomprises a rotatable supporting roller in which rib-shaped reinforcinggrooves are formed in a direction of a generating line of a cylinder.29. The fixing apparatus according to claim 23, wherein a circumferenceof the endless belt is a non-integral multiple of an outer circumferenceof the supporting roller.
 30. The fixing apparatus according to claim23, wherein the belt supporting member comprises a rotatable supportingroller in which knurl-shaped projections and depressions are formed onan outer surface of a cylinder.
 31. The fixing apparatus according toclaim 30, wherein: the projections and depressions are formed with apredetermined pitch in a circumferential direction of the supportingroller; and a circumference of the endless belt is a non-integralmultiple of a pitch of the projections and depressions.
 32. The fixingapparatus according to claim 23, wherein the belt supporting member isformed with a supporting roller in which a plurality of channel-shapedsheets are combined into a cylindrical shape.
 33. The fixing apparatusaccording to claim 23, wherein the belt supporting member is formed witha guide member in which a sheet is formed into an arc shape.
 34. Animage forming apparatus comprising the fixing apparatus according toclaim
 1. 35-38. (canceled)